This invention relates to a method of and plant for the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapor such that the ascending stream of vapor is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
It is known to separate air in a double rectification column comprising a higher pressure column which receives a first stream of purified, compressed, vaporous air at a temperature suitable for its separation by rectification, and a lower pressure column which receives a stream of oxygen-enriched liquid air for separation from the higher pressure rectification column, and which is in heat exchange relationship with the higher pressure rectification column through a condenser-reboiler, of which the condenser provides liquid nitrogen reflux for the separation and the reboiler provides an upward flow of vapor in the lower pressure column.
A double rectification column may be operated so as to produce a liquid oxygen fraction at the bottom of the lower pressure column and a vaporous nitrogen fraction at the top of the lower pressure column. The oxygen fraction may be essentially pure, containing less than 0.5 per cent by volume of impurities, or may be impure containing up to 50 per cent by volume of impurities.
There is a net requirement for refrigeration to be provided to the air separation plant. At least part of this requirement arises from the operation of the double rectification column at cryogenic temperatures. Particularly if none of the products of the air separation is taken in liquid state, the requirements for refrigeration are typically met by raising the pressure of a second stream of compressed air to at least two bar above the operating pressure at the top of the higher pressure column and expanding it with the performance of external work in an expansion turbine which exhausts into the lower pressure column. Typically, the turbine is coupled to a booster-compressor which raises the pressure of the air to above that at the top of the higher pressure column.
U.S. Pat. No. 5,237,822 discloses an air separation process employing two extension expansion turbines, one exhausting into the lower pressure column, and the other exhausting into the higher pressure column. The former turbine has the same inlet pressure as the higher pressure column.
U.S. Pat. No. 5,511,381 discloses a similar air separation process, but with both turbines having inlet pressures higher than the inlet pressure of the higher pressure column.
In both of the above patents the air separation process includes forming a third compressed air stream at a higher pressure than the other air streams. The third compressed air stream is employed to vaporise an oxygen product stream, it expanded and is introduced into the double rectification column in liquid state.
U.S. Pat. No. 5,586,451 discloses with reference to FIG. 2 a process in which a single air stream is substituted for the aforementioned first and third air streams. The single air stream is compressed to a higher pressure than the second air stream, is expanded, and is introduced into the higher pressure column in partially condensed state. Most of the air, therefore, has to be compressed to a pressure substantially above the operating pressure of the higher pressure column.
U.S. Pat. No. 5,537,570 provides examples of a further kind of air separation plant. There is a first condenser-reboiler which condenses a part of the top nitrogen fractions separated in the higher pressure column. The condensation is effected by indirect heat exchange with a stream of the bottom oxygen-enriched liquid fraction formed in the higher pressure column. As a result, the stream of the bottom oxygen-enriched liquid fraction is partially reboiled. Resulting vapor and residual liquid are fed to the lower pressure column. The plant employs a single generator-loaded expansion turbine exhausting into the lower pressure column. The air to be separated is compressed in a main, plural stage, compressor. The main air feed to the higher pressure rectification column is taken from a lower pressure stage than the feed to the expansion turbine.
An air separation plant typically consumes a considerable amount of power. It is therefore desirable for the air separation plant to have a configuration which enables power consumption to be minimised without unduly increasing its capital cost. In order to minimise the power consumption much attention in the art has been recently focused upon operating the lower pressure column with two reboilers, one operating at a higher temperature and being heated by a flow of the air to be separated, and the other operating at a lower temperature and being heated by a flow of nitrogen separated in the higher pressure column. A disadvantage of such a plant is that the requirement for a second reboiler adds to its complexity and capital cost.